How Scientists Hunted Down the Oldest Oxygen in the Universe

13.1 billion light-years away sits a galaxy known as SXDF-NB1006-2. When scientists trained the Atacama Large Millimeter/submillimeter Array (ALMA) on it, they made the most distant detection of oxygen ever.

Akio Inoue, the lead author of the new research, explains why astronomers look at these distant galaxies. “Seeking heavy elements in the early Universe is an essential approach to explore the star formation activity in that period,” said Inoue. “Studying heavy elements also gives us a hint to understand how the galaxies were formed and what caused the cosmic reionization.”

Before planets, stars and galaxies began to take shape; the Universe was filled with electrically neutral gas. Several hundred million years after the Big Bang, the first stellar objects began to shine. This phase is known as cosmic reionization. And it broke up neutral atoms and ionized the gas.

Astronomers from across the world continue to debate exactly what kind of objects are responsible for this phase. Training the most powerful telescopes on Earth to the most distant galaxies helps us get closer to an answer.

When SXDF-NB1006-2 was first discovered in 2012, it set the record for the most distant galaxy ever observed. That record didn’t stand long and continues to be broken each year. But the galaxy was still a great candidate to search for heavy elements such as oxygen and carbon.

Observing SXDF-NB1006-2 with ALMA brought its own non-traditional challenges. Astronomers from across the world vie for time with the best telescopes. Inoue and the rest of the team turned to computer simulations to predict how easily an oxygen detection would be for ALMA to see. The results showed the oxygen emission should be easily detected.

With the coveted time to use ALMA secured, the astronomers observed the galaxy and detected just what they predicted – light from ionized oxygen. And with it, concrete evidence that oxygen existed just 700 million years after the Big Bang.

Light from ionized oxygen detected by ALMA is seen in green.

Their simulations even predicted how much oxygen would be detected. Just 10% of the oxygen seen in our own sun. Naoki Yoshida at the University of Tokyo explains that’s normal. “The small abundance is expected because the Universe was still young and had a short history of star formation at the time.” But Yoshida says they found something they didn’t expect – a “very small amount of dust.

The astronomers aren’t sure what to make of the lack of dust, but it would allow the intense ultraviolet light from massive stars to ionize huge amounts of gas outside the galaxy.

“SXDF-NB1006-2 would be a prototype of the light sources responsible for the cosmic reionization,” said Inoue.

SXDF-NB1006-2’s story is far from over. More observations from ALMA are already underway. And the next generation of telescopes will only provide better resolutions. “Higher resolution observations will allow us to see the distribution and motion of ionized oxygen in the galaxy and provide vital information to help us understand the properties of the galaxy,” says Yoichi Tamura of the University of Tokyo.

As each year goes by, astronomers will continue to reveal the secrets hiding in the darkest and farthest reaches of the Universe.

The ALMA telescope

I couldn’t end this post without talking a bit more about ALMA. It’s not just one telescope. 66 radio antennas (most of them measuring 39 feet in diameter) make up the array dotting the Atacama desert in northern Chile.

Many of the world’s most powerful telescopes rest at high altitudes. And ALMA is one of the highest at 16,570 feet above sea level. Combine the high altitude with one of the driest places on Earth and nearly every night is perfect for observations.

The arid landscape also means it’s not densely populated. Which is a big plus for radio telescopes since they don’t have to deal with radio interference from cities and towns.

ALMA has been fully operational since 2013 and three years later, continues to unlock the secrets of the early Universe.